To: rsgb_lf_group@blacksheep.org
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 <5B44A64A.1070307@posteo.de>
From: N1BUG <paul@n1bug.com>
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Message-ID: <3d737f55-9e30-1c39-f34c-ea6f3c1c06af@n1bug.com>
Date: Tue, 10 Jul 2018 09:05:34 -0400
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In-Reply-To: <5B44A64A.1070307@posteo.de>
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Subject: Re: LF: Receiver protection
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Reply-To: rsgb_lf_group@blacksheep.org

Hi Stefan,

>> Now I have a pair of back to back 1N4148 diodes across the receiver
>> front end but I don't think this offers real protection because they
>> allow too high voltage before conducting.
> I don't think so. They work until several 100 MHz so they are extremely 
> fast.

I am thinking they only limit peak voltage to 0.6V? If that's right,
then I think this is about +5 dBm, perhaps still enough to damage a
receiver.

> Further you can switch a small R (depending on the design) in 
> front of the diodes. This also builds a RC lowpass which can help to 
> rise the performance.

I was trying to avoid switching because with several different
transmitters or exciters the control for this becomes a lot of work.
If I do go with some kind of switching, I will just disconnect the
antenna from the receiver completely while transmitting (and connect
a 50 ohm termination at the receiver input). That is probably the
best way. I need to stop being lazy. ;-)

> I'm also using 1N4148 diodes but not back to back, rather one against 
> ground, the other against +, while the center is at V/2.

I need to think about this. I am not sure I understand how this works.

>>   Also there is a preamp
>> after the didoes which can bring the RF to dangerous levels!
>>    
> How? The output voltage of the preamp can't be higher than the supply 
> voltage.

Sorry, I mean dangerous to the receiver. If I have +5 dBm at the
input of a 2N5109 operating at 13V I think there is high risk for
what follows the preamp!

I tried putting another pair of 1N4148 diodes after the preamp but
this did not work. Somehow it caused the system to go into
oscillation or a permanently clamped off state. It was OK until my
first transmission, but after my TX stopped there was no RX until
power to the preamp was cycled off, then on again. I do not
understand what was happening there.

By the way, I bought all the parts for the automatic vario steering
circuit but there are so many projects I did not build it yet.

73,
Paul